1. Technical Field
This application is related to energy absorbing materials suitable for armor against projectiles, shape charges, EFPs, and explosives.
2. Related Technology
Effective armor technologies have been sought for many decades to protect humans, vehicles, and systems against projectile weapons and explosive blasts.
The Air Force Research Laboratory has increased blast resistance of infill composite masonry unit walls by applying an elastomeric coating to the surface of the wall. As described in Porter, J. R., Dinan, R. J., Hammons, M. I., and Knox, K. J., “Polymer coatings increase blast resistance of existing and temporary structures”, AMPTI AC Quarterly, Vol. 6, No. 4, pp. 47-52, 2002, the elastomeric coating is a two-component sprayed-on polyurea, and the coating can be applied to the interior and exterior surfaces of the wall, or to only one surface. It functions primarily by reducing fragmentation (flying debris) of the structure destroyed by the blast.
Composite polyurea coatings have been tested for mitigating the damage from ballistic fragmentation and projectiles. For example, Tekalur, S. A, Shukla, A., and Shivakumar, K., “Blast resistance of polyurea based layered composite materials”, Composite Structures, Vol. 84, No. 3, pp. 271-81, (2008) discloses test results for layered and sandwiched layers of polyurea and E-glass vinyl ester.
Bogoslovov, R. B., Roland, C. M., and Gamache, R. M., “Impact-induced glass transition in elastomeric coatings”, Applied Physics Letters, Vol. 90, pp. 221910-1-221910-3, 2007, which is incorporated by reference herein in its entirety, discloses coating steel with a polybutadiene or polyurea elastomeric layer for impact loading, and compares their failure mechanisms.
Possible mechanisms contributing to the blast and ballistic mitigation of composites are discussed in Xue, Z. and Hutchinson, J. W., “Neck development in metal/elastomer bilayers under dynamic stretchings”, International Journal of Solids and Structures, Vol. 45, No. 3, pp. 3769-78, (2008); in Xue, Z. and Hutchinson, J. W., “Neck retardation and enhanced energy absorption in metal-elastomer bilayers”, Mechanics of Materials, Vol. 39, pp. 473-487, (2007); and in Malvar, L. J., Crawford, J. E., and Morrill, K. B.; “Use of composites to resist blast”, Journal of Composites for Construction, Vol. 11, No. 6, pp. 601-610, (November/December 2007).
A. Tasdemirci, I. W. Hall, B. A. Gama and M. Guiden, “Stress wave propagation effects in two- and three-layered composite material”, Journal of Composite Materials, Vol. 38, pp. 995-1009, (2004), discloses tests on a three layered composite material with a layer of EPDM rubber between an alumina tile and a glass epoxy composite plate.
Information on the material properties of viscoelastic materials is found in D. I. G. Jones, Handbook of Viscoelastic Vibration Damping, Wiley, 2001, pp. 39-74.
A review of mechanical behavior of viscoelastic materials can also be found in R. N. Capps, “Young's moduli of polyurethanes”, J. Acoustic Society of America, V. 73, No. 6, pp. 2000-2005, June 1983. In discussing Capps's FIG. 2, Capps discloses that viscoelastic material has four general regions of mechanical behavior: a low temperature, glassy region in which the storage modulus is almost constant; a glass-rubber transition region in which the storage modulus changes remains more or less the same; a rubbery region in which the value of the modulus remains more or less the same; and a flow region in which the values of the modulus drops very rapidly. The behavior in this region is greatly influenced by the molecular weight. For viscoelastic materials, typically the loss tangent is almost constant in the rubbery region, increasing slightly with increasing frequency or decreasing temperature. The onset of the glass-rubber transition can be characterized by a peak in the loss tangent. The loss tangent then decreases until it reaches another plateau, where the loss tangent is again almost constant. The material is then in the glassy region, in which the material has a high storage modulus and a low loss tangent.